Power supply for imaging systems are generally referred to as scan room power supplies. Prior art scan room power supply (SRPS) systems may include some form of protection network including voltage sensors to sense the voltage levels and turn the system down completely. A three-phase input power supply supplies power to the SRPS. Further, there are three MOSFETs per each phase and hence a minimum of three voltage sensors and an elaborate analog network is required with software interrupts to detect, communicate and shut down the imaging system.
The performance of the imaging system depends on MOSFET characteristics like avalanche breakdown ratings, which vary widely between MOSFETs. During dynamic turn ON the MOSFETs experience higher voltage level across the drain to source terminals based on the point on wave of the input sinusoidal waveform of the input supply. When this exceeds the absolute maximum voltage rating specification or the maximum avalanche voltage specification, the MOSFETs typically fail.
Failure of the MOSFETs in SRPS happens even under phase loss condition, which is absence of a single phase out of the three-phase supply that supplies power to the SRPS. Invariably, the control power is derived from one of the three phases. If this phase is lost, then the control power is lost which leads to the MOSFETs never getting turned ON. This leads to the MOSFETs experiencing the full level supply voltage. The capability of the MOSFET to survive under these conditions is solely dependant on the avalanche voltage specifications, which is a short time rating for the MOSFET. When the voltage across the MOSFET exceeds the avalanche voltage specification, the MOSFET may fail resulting in failure of the SRPS thereby causing unnecessary system downtime in MR scanning leading to patient discomfort.
Several solutions have been suggested in the prior art. One of the solutions describes using overvoltage sensors and overvoltage analog circuitry. However, the overvoltage sensors and the overvoltage analog circuitry consume significant board space while being economically disadvantageous.
Another prior art describes using snubber circuits to protect the MOSFETs from turn off overvoltage. The disadvantage associated with using the snubber circuits is the snubber circuits may prevent MOSFET failure but cannot prevent system downtime.
Further, in the prior art systems, during turn off due to the overvoltage across the MOSFET, the power dissipation due to the turn off loss is higher, mandating MOSFETs with high power dissipation capacities. This requires higher investment in heat sink and MOSFETs.
Hence there exists a need for a power supply for a magnetic resonance imaging system that prevents MOSFET failure and thereby prevents unnecessary system downtime during MR scanning.